Higgs phenomenology and new physics beyond the standard model

Department: University of Delaware, Department of Physics and Astronomy

Publisher: University of Delaware

Date Issued: 2015

Abstract: The existence of the Higgs boson was predicted in the 1960's. The discovery of the Higgs boson in 2012 at the Large Hadron Collider (LHC) has been a remarkable triumph of the Standard Model (SM) and particle physics. However, there are still fundamental questions that cannot be answered by the SM. A variety of extensions to the SM have been proposed to explain these mysteries. In this thesis we explore the Higgs boson mass in several extensions to the SM. We first study the impact of vectorlike fermions on the SM Higgs mass bounds. The presence of these fermions significantly modifies the vacuum stability and perturbativity bounds on the mass of the SM Higgs boson. The new vacuum stability bound in this extended SM is estimated to be 117 GeV, to be compared with the SM prediction of about 129 GeV. The non-minimal gravitational coupling ξ H [dagger]HR between the SM Higgs doublet H and the curvature scalar R opens up a very intriguing scenario for inflationary cosmology. In the presence of this coupling, the effective ultraviolet cutoff scale is given by Λ [approximate] mP /ξ, where mP is the reduced Planck mass, and ξ > 1 is a dimensionless coupling constant. In type I and type III seesaw extended SM, we investigate the implications of this non-minimal gravitational coupling for the SM Higgs boson mass bounds based on vacuum stability and perturbativity arguments. A lower bound on the Higgs boson mass close to 120 GeV is realized with type III seesaw and ξ ∼ 10 - 10 3 . Supersymmetry is by far the most compelling extension of the SM. We consider extensions of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) in which the observed neutrino masses are generated through a TeV scale inverse seesaw mechanism. The new particles associated with this mechanism can have sizable couplings to the Higgs field which can yield a large contribution to the mass of the lightest CP-even Higgs boson. With this new contribution, a 126 GeV Higgs is possible along with order of 200 GeV masses for the stop quarks for a broad range of tan \beta. Finally we study the implications of the inverse seesaw mechanism on the sparticle spectrum in the Constrained Minimal Supersymmetric Standard Model (CMSSM) and Non-Universal Higgs Model (NUHM2). Employing the maximal value of the Dirac Yukawa coupling involving the up type Higgs doublet provides a 2-3 GeV enhancement of the lightest CP-even Higgs boson mass. This effect permits one to have lighter colored sparticles in the CMSSM and NUHM2 scenarios with LSP neutralino, which can be tested at LHC14.